In general, an integrated circuit refers to an electrical circuit contained on a single monolithic chip containing active and passive circuit elements. As should be well understood in this art, integrated circuits are fabricated by diffusing and depositing successive layers of various materials in a preselected pattern on a substrate. The materials can include semiconductive materials such as silicon, conductive materials such as metals, and low dielectric materials such as silicon dioxide. The semiconductive materials contained in integrated circuit chips are used to form almost all of the ordinary electronic circuit elements, such as resistors, capacitors, diodes, and transistors.
Integrated circuits are used in great quantities in electronic devices such as digital computers because of their small size, low power consumption and high reliability. The complexity of integrated circuits ranges from simple logic gates and memory units to large arrays capable of complete video, audio and print data processing. Presently, however, there is a demand for integrated circuit chips to accomplish more tasks in a smaller space while having even lower operating voltage and power requirements.
Currently, the semiconductor industry is focusing its efforts on reducing dimensions within each individual integrated circuit in order to increase speed and to reduce energy requirements. The demand for faster and more efficient circuits, however, has created various problems for circuit manufacturers. For instance, a unique problem has emerged in developing equipment capable of characterizing, evaluating and testing faster chips. Timing errors and pulse width deviations may constitute a greater portion of a signal period at higher frequencies. As such, a need exists not only for devices capable of detecting these errors but also devices capable of characterizing and identifying these critical timing deviations.
In the past, electronic measurement devices have been used to test integrated circuits for timing irregularities by making frequency and period measurements of a signal output from the circuit. Certain devices, known as time interval analyzers or time counters, can perform time interval measurements, i.e. measurements of the time period between two input signal events, or can obtain other time characterizations of an input signal. A signal timing event is typically defined as the specific instant in time at which an input signal reaches a certain predefined level, also known as the threshold voltage level. At the specific time when the input signal crosses the threshold voltage level, a signal timing event occurs.
A time interval analyzer generally includes a continuous running clock and a continuous event counter. Typically, the device includes one or more timing measurement circuits on each of a plurality of measurement channels. Each measurement channel receives an input signal. By directing the signal across the channel to a given number of measurement circuits, known as interpolators, the device is able to measure the time interval between two events in the signals. Such devices are capable of making millions of measurements per second.
An alternative device for measuring timing parameters is a counter-based system. Similar to some extent to the time interval analyzer, a counter-based system measures the time period between two signal events using a clock that starts and then stops upon the respective signal events.
By measuring certain characteristics of a signal emitted by an integrated circuit, time interval analyzers and counter-based measurement devices can be used to detect timing errors or deviations that may be present within the circuit. This information can then be used to assist in developing an integrated circuit or for detecting defects in mass-produced circuits.
Timing fluctuations in integrated circuit signals are generally referred to as xe2x80x9cjitterxe2x80x9d. Jitter, broadly defined as a timing deviation between a real pulse train and an ideal pulse train, can be a deviation in phase and/or pulse width. Jitter typically refers to small variations caused by supply voltage fluctuations, control-system instability, temperature variation, noise and the like.
Instruments such as time interval analyzers, counter-based measurement devices and oscilloscopes have been used to measure jitter. In particular, time interval analyzers can monitor frequency changes and frequency deviation over time. In this manner, they not only detect jitter, but can also characterize jitter so that its source can be determined.
Further, devices such as time interval analyzers may be used to monitor single-ended or differential signals. Generally, single-ended signals are carried on a single cable and are referenced to ground or some other fixed voltage. Differential signals are carried on two cables and are referenced against each other. It may often be the case that the two signals are complements of each other. These two types of signals, single-ended and differential, have typically been monitored by distinctly configured pieces of equipment since each signal type requires a different type of input circuit and number of cables to detect it. Past measurement devices have thus typically been hard-wired during their manufacture for measurement of either single-ended or differential signals.
One possible way to switch between single-ended and differential signal inputs is by using a combination of electromechanical relays at the input to a time measurement device. However, relays within a signal path tend to introduce undesired capacitance to the signal path and often degrade high frequency performance of testing equipment. Thus, testing equipment with a minimum number of electromechanical relays in the input signal paths is highly desired.
As integrated circuits have grown more advanced, the need for differential signal measurements has grown. At the same time, the need to maintain the capability for single-ended measurement has remained. As a result, there is a need for a device capable of switching between single-ended and differential measurement modes that minimizes the number of component parts, as well as the number of relays in the path of the input signal.
In addition, most output signals from older generation integrated circuits emitted older digital signals where a binary xe2x80x9c0xe2x80x9d was a voltage between about 0 to 0.7 and binary xe2x80x9c1xe2x80x9d was about 4 to 5 volts. To test such signals, a simple single-ended input circuit with a single termination resistor to ground (0 volts) was used. In accordance with more modem technologies, output signals now exist with different voltage levels to represent logical xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d. For example, an LVDS output has logical xe2x80x9c0xe2x80x9d of 2.4 volts and logical xe2x80x9c1xe2x80x9d of 3.0 volts. In this case, the ideal input circuit would comprise a termination resistor connected to the range between logical xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d, such as 2.7 volts. So it is desired, for automatic test equipment to have an input termination voltage that is programmable by the user.
In addition, certain differential signals often need to be analyzed in a variety of fashions. One measurement mode involves comparing a differential signal against another differential signal such that the time difference between respective cross-over points can be determined. Another measurement mode involves measuring corresponding portions of a single signal in a differential pair to characterize rise time, fall time, undershoot and overshoot. Since individual signals in a differential pair may typically be related to each other, additional characterization of the levels of similarity between the two signals in the pair may also be desired. Due to the many types of desired measurements, it is preferred to have a time measurement unit, such as a time interval analyzer or a time counter, that is capable of measuring a differential signal in a differential fashion, and immediately thereafter in a single-ended fashion with a very close time interval between the two types of measurements. Such an application imposes the need to be able to switch the measurement input circuit from one measurement type to the next very quickly, preferably on the order of less than a few microseconds.
In view of the recognized features encountered in the prior art and addressed by the present subject matter, specialized measurement input circuitry has been developed. In general, such measurement circuitry provides a way to enable a single time measurement device to measure characteristics of both single-ended and differential input signals. This is preferably achieved in accordance with the disclosed circuitry via software-implemented toggling of different measurement modes at the direction of a user. Toggling between different measurement modes is preferably achieved in accordance with the present technology within a time no greater than a few microseconds. Varied exemplary embodiments of such measurement circuitry are hereafter presented, selected of which offer such advantages as minimized signal degradation and reduced component part.
One exemplary embodiment of the present subject matter relates to a time measurement device capable of measuring both single-ended and differential signal inputs. The time measurement device preferably includes measurement circuitry for obtaining timing information about selected input signals as well as input circuitry for selecting the input signals for which to obtain timing information. The input circuitry preferably comprises a plurality of dual-input, single-output comparators and a plurality of software driven selection devices. Input signals are selectively provided to both comparator inputs and a comparator output signal is provided from each of the plurality of comparators. Each selection device receives a comparator output signal from each of the dual-input, single-output or complementary dual output comparators and subsequently outputs a selected signal to the measurement circuitry based on software selection inputs to the selection devices.
The aforementioned input circuitry is preferably utilized to enable the measurement of single-ended and differential input signals by time measurement circuitry. More particular exemplary embodiments of the input circuitry correspond to either three-comparator or five-comparator configurations. Additional resistive networks may be provided before each comparator to isolate an input signal from input impedance of other comparators in the input circuitry. The selection devices which receive outputs from each comparator may preferably comprise multiplexors. In such case, the multiplexor input signals include an output signal from each comparator and the multiplexor control signals correspond to software selection inputs.
Embodiments of the disclosed input circuitry may be provided as the input to measurement channels in a time interval analyzer. The input circuitry receives an input signal and converts it to a timing signal based in part on the software selection of measurement mode types. Each measurement channel contains at least one interpolator for receiving the timing signal and obtaining a measurement corresponding to a selected transitions within its received timing signal. The resultant measurement information obtained by the interpolator can be directed to other components in the time interval analyzer for proper recording and storing of the measurement information.
Additional embodiments of the present subject matter concern methodology in accordance with obtaining both single-ended and differential signal measurements. One exemplary embodiment of such methodology concerns a selection method for determining whether measurement circuitry is to obtain timing measurements corresponding to single-ended input signals or to differential input signals. This may be done by providing programmable threshold voltages and, when necessary, programmable termination voltages to the input signal sources. More particularly, a first step in such a selection method is to selectively provide at least two input signals to a plurality of comparators, wherein each comparator is characterized by first and second inputs and an output. A plurality of programmable voltage sources may then be provided to each first and second input of the plurality of comparators that are not selectively connected to one of the two input signals. A plurality of selection devices is also provided, each for receiving comparator outputs from selected comparators and for receiving user-defined input for determining which of the received comparator outputs will be sent to the measurement circuitry such that timing measurements related to the comparator outputs can be obtained. Depending on the type of signal to be measured, the voltage sources are used for either termination or threshold purposes without the need for any electromechanical switches.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the present subject matter, and together with the description serve to explain certain principles of the disclosed technology. Additional embodiments of the present subject matter may incorporate various steps or features of the above-referenced embodiments, and the scope of the presently disclosed technology should in no way be limited to any particular embodiment. Additional objects, features and aspects of the present subject matter and corresponding embodiments are discussed in greater detail below.